Annabeth might be initially shocked and a bit flustered. She's a composed person, but this unexpected act from Percy would catch her off - guard. Percy, on the other hand, might be a bit proud of his 'mark' on Annabeth at first. Their friends like Grover could be really surprised and might tease them about it, while some of the more serious demigods might be a bit disapproving, thinking it's not appropriate behavior for heroes.
Annabeth would probably blush furiously and try to cover up the hickey. She might scold Percy a little for being so bold in public or at least in a place where they could be easily discovered. Percy would likely realize the consequences of his actions a bit too late and then try to make it up to Annabeth. Their friends would have a mix of reactions. Some would be excited and giggle about it, while others, especially those with more traditional values, would be a bit taken aback and might give Percy a talking - to.
Well, Annabeth is a strong - willed character. She might initially be angry with Percy for giving her a hickey as it could be seen as a bit of a possessive act. But then she might also secretly like the idea that Percy is so into her. Percy would be worried about Annabeth's reaction at first but also be a bit defiant, thinking it was just a natural expression of his love. Their friends' reactions would vary widely. Some would be supportive and think it's just a cute couple thing, while others would be worried about how it reflects on their group as a whole, especially in the context of their roles as demigods and leaders among their peers.